At the start of a battle, each combatant makes an initiative check. An initiative check is a Dexterity check. Each character applies his or her Dexterity modifier to the roll, as well as other modifiers from feats, spells, and other effects. Characters act in order, counting down from the highest result to the lowest. In every round that follows, the characters act in the same order (unless a character takes an action that results in his or her initiative changing).
If two or more combatants have the same initiative check result, the combatants who are tied act in order of total initiative modifier (highest first). If there is still a tie, the tied characters should roll to determine which one of them goes before the other.